Basemap does not respects False Easting or False Northing

While creating a basemap from an EPSG code with the x_0 or y_0 (False Easting and False Northing), the Basemap instance does not respect those values. I tried with 'tmerc' projections only because it's of my interest. I leave an example:

Suppose I want to use this EPSG basemap:

```
# POSGAR 94 / Argentina 1
<22181> +proj=tmerc +lat_0=-90 +lon_0=-72 +k=1 +x_0=1500000 +y_0=0 +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs <>
```

It can be found in "basemap/data/epsg" file.

Then I create the basemap and print its parameters:

```
from mpl_toolkits.basemap import Basemap

lon_min, lat_min = -75, -45
lon_max, lat_max = -70, -40
bm = Basemap(epsg=22181,
llcrnrlon=lon_min, llcrnrlat=lat_min,
urcrnrlon=lon_max, urcrnrlat=lat_max)

print bm.proj4string
```

I get the following output:
`+a=6378137.0 +b=6356752.31425 +k_0=1.0 +y_0=-5012640.49452 +lon_0=-72.0 +proj=tmerc +x_0=236540.642361 +units=m +lat_0=-90.0`

As we can see, the Basemap overrides the false easting and false northing information:
**`+x_0=236540.642361 +y_0=-5012640.49452`**

It would be great that the Basemap respect those values and even better if they can be passed as arguments while initializing it in the usual way (without using EPSG codes), for example:

```
bm = Basemap(projection='tmerc',
lon_0=lon_0, lat_0=lat_0,
x_0=x_0, y_0=y_0
llcrnrlon=lon_min, llcrnrlat=lat_min,
urcrnrlon=lon_max, urcrnrlat=lat_max)
```

Start at the Basemap constructor and EPSG lookup using the entry for 22181 in basemap/data/epsg; reproduce the tmerc example and inspect how projection parameters become proj4string. Confirm that EPSG-provided x_0 and y_0 are retained and determine how equivalent constructor arguments should be handled. Done means the example preserves both false offsets and regression coverage verifies them.

Written by the indexing model from the issue text.