Added some structure to automated tests

Now tests for different areas go in different namespaces and the
MegaClient tests are further split apart by functionality area.
Added more file transfer tests

Now also testing upload/download of various sizes of files, with
integrity verification. Just to catch any weird edge cases in the
algorithms code.
Improved API error handling

Due to the inconsistent ways Mega API is providing result codes, the API
client was not catching all the cases that it should.

Previously, it was always expecting a list of codes, e.g. [-1, -1] but
now it also accepts a single code as a value e.g. -3.
Added note about slow tests
Improved API error handling

For real this time! JSON.net threw me off the right path with last
related commit.
Disabled the very slow FileTransferTests

They still work. It is just that they are too slow for day to day usage.