Expand dump_ram to dump_memory

From slack:

Andrew Wu is putting together an external SPI flash dumping routine that can be loaded into RAM on a controller tile and will let you dump the contents of external SPI flash programmatically. It works similar to how the `jlink` flashing routines work by injecting a blob of code into ram, manually setting the PC to the RAM and then interacting with the `agent` it just injected.

I want to update the `debug_manger` in `iotile-core` to be able to expose this ability now and propose the following change:
- rename `dump_ram` to `dump_memory` and have it take a `region` name (`raw`, `ram` or `external`), an `offset` and a `length`. This also fixes your need (Luke) to be want to dump registers as well. `raw` would just look at the address space of the chip and let you do whatever, `ram` would look for offsets in RAM, `external` would be defined by the `jlink-transport` plugin but mean download the spi flash.