Negative value as accuracy of table.

While testing I have faced a case where `table.accuracy` is negative number.

PDF:[page-3.pdf](https://github.com/camelot-dev/camelot/files/3455388/page-3.pdf)
Code:
```
tables=camelot.read_pdf('/Users/skatipomu/Table_Extraction_Camelot/page3.pdf',pages="all)
[table.accuracy for table in tables]
```
Output:
`[99.99999999999997, -20.852716930856104]`

I think the reason is because in `compute_accuracy` method in utils.py while calculating accuracy we are subtracting error percentage from 1. It is supposed to be in the range [0.0,1.0] but the errors passed on to this method contains error percentages in the range[0 to 100] which inturn is from `get_table_index` method. So dividing this error by 100 solved the issue for me.

```
def compute_accuracy(error_weights):
"""Calculates a score based on weights assigned to various
parameters and their error percentages.

Parameters
----------
error_weights : list
Two-dimensional list of the form [[p1, e1], [p2, e2], ...]
where pn is the weight assigned to list of errors en.
Sum of pn should be equal to 100.

Returns
-------
score : float

"""
SCORE_VAL = 100
try:
score = 0
if sum([ew[0] for ew in error_weights]) != SCORE_VAL:
raise ValueError("Sum of weights should be equal to 100.")
for ew in error_weights:
weight = ew[0] / len(ew[1])
for error_percentage in ew[1]:
**score += weight * (1 - error_percentage)**
except ZeroDivisionError:
score = 0
return score
```
from `score += weight * (1 - error_percentage)` to `score += weight * (1 - error_percentage/100.0)`

Start in utils.py at compute_accuracy and trace the error values supplied by get_table_index, then reproduce the negative result with page-3.pdf. Done means the reported table accuracy no longer becomes negative and the calculation matches the documented score range.

Written by the indexing model from the issue text.