Add API to access the process id of the started process

Added new agent message "GetProcessId" and a API "winpty_get_process_id"
to allow access to the process id of the started process. The process id
is needed to integrate "winpty" with Eclipse CDT to launch native Windows
console applications inside the Eclipse UI.
Simplify patch by using GetProcessId

GetProcessId simplifies the code as it avoids to add the additional member variable 'm_childProcessId'. Cleaned up the indentation.
Fixed indentation

Fixed indentation by replacing TABs by 4 spaces.
Merge pull request #14 from ustieber/patch-1-GetProcessId

Add API to access the process id of the started process