Remove common.hh

The common.hh header file is currently not very useful and probably will
never be. When a common header is needed in the future, that will be
added in the sesh::common namespace.
Add Char.hh and String.hh headers

Using the std::wstring type directly makes it harder to change the
underlying character type in the future. The new header provides the
Char and String types that abstracts the character type so that it can
be changed more easily.
Use sesh::common::String rather than std::wstring
Use sesh::common::Char rather than wchar_t
Rename toWstring to toString
Add OutputStream.hh and StringStream.hh headers
Use OutputStream and OutputStringStream

rather than wostream and wostringstream.
Merge pull request #8 from magicant/refactor/common

Replace common.hh with sesh::common namespace