set shadow context properly, fix malloc pointer tracking

shadow context was not set properly for all system_* functions,
meaning there were cases where malloc was getting called and
intercepted and we thought it was the plug-in calling it instead
of shadow. this has been updated to function correctly.

malloced pointers are now removed from the hash table on free

refs #144
added ram tracking to tracker

malloc family will now be intercepted by default, but will only track
ram usage if the heartbeatloginfo string contains 'ram', either
set globally on the cli or for individual nodes via xml.

refs #144
remove memtracker option

refs #144
Merge branch 'master' of github.com:shadow/shadow