Ability to set specific log levels and store debug header as file

New command that allows user to set the debug category and level per http://www.salesforce.com/us/developer/docs/apexcode/Content/sforce_api_header_debuggingheader.htm

Stores the debug header as an external xml file so settings are saved. Allows specifying of ALL or specific categories and levels, and command to show current settings. soapExecute in partner.go updated to read the header from file, for both apex execute and future run tests call. Handles case, handles the xml file not yet existing. 

This is a proof of concept, I have some outstanding questions to discuss, including structure, where to store the header.xml file and probably more. 

Examples: 

force loglevels ALL:INFO
force loglevels APEX_CODE:ERROR APEX_PROFILING:INFO WORKFLOW:FINEST
force loglevels show 
APEX_CODE:ERROR
APEX_PROFILING:INFO
WORKFLOW:FINEST