more standard thrift sdk module generator

this fixed the bug that previous `thriftpy.load` generated TPayload
objects could not be pickled.

the TPayload objects can be picked now if module_name provided or
when sdk was imported with import hook enabled.
Merge pull request #22 from noodle4u/load_module, close #22

more standard thrift sdk module generator
coding style refines
add tests for load and load_module
module_name should be unique

same module_name will always be treated as same module. so the pickled
bytes can be used across multiple systems as long as the module names
are the same.