Thrift: Support for explicit Python module declaration.

Summary:
Previously, Thrift used the name of the .thrift file as the python module name.
This wasn't very flexible.  Now the python module can be explicitly declared.
Also, there was no need for t_py_generator to inherit from t_oop_generator.

Reviewed By: mcslee

Test Plan:
cd test/py/explicit_module
./runtest.sh

Revert Plan: ok


git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665234 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/compiler/cpp/src/thriftl.ll b/compiler/cpp/src/thriftl.ll
index d047acc..fbad955 100644
--- a/compiler/cpp/src/thriftl.ll
+++ b/compiler/cpp/src/thriftl.ll
@@ -72,6 +72,7 @@
 "cpp_type"       { return tok_cpp_type;       }
 "java_package"   { return tok_java_package;   }
 "php_namespace"  { return tok_php_namespace;  }
+"py_module"      { return tok_py_module;      }
 "perl_package"   { return tok_perl_package;   }
 "ruby_namespace" { return tok_ruby_namespace; }
 "xsd_all"        { return tok_xsd_all;        }