Better mirror path calculation.

- Add support in spack.url for extrapolating actual file type for URL
- Move mirror path computation to mirror.py from package.py
add gperftools (tcmalloc and friends)
Allow fake installations (just make the directory).

- Use for debugging.
Fix for SPACK-49.

- name conflict in imp.load_source caused this to fail.
- Python modules loaded by imp have unique names now.
Merge branch 'bugfix/load-hooks-fix' into develop
Fix for SPACK-50

Bad format string in version check.
Merge branch 'features/gperftools' into develop
Tweak extrae indentation.
Merge branch 'features/better-mirror-support' into develop
Initial versions of Qt and some dependencies.
gnutls, nettle, wget, dbus
Updates for Qt dependencies
Print out fetch, build, and total time for builds.
Add patch function to Package, so that packages can define custom patch functions.
spack env command

spack env allows regular commands to be run with a spack build environment.
It also displays the spack build environment for a package.
Clearer code in filter_file.
Qt4 builds successfully with proper RPATHs.