General manual overhaul in the system sections, including:

* Updates to the Ubuntu install procedures
* Introduction of sections for installing on other Linux variants
* General overhaul of the build-from-source method including a new
    section for gmsh.
* Updated recommended petsc configuration options.
* Updates for the migration from bzr+launchpad to git+github
Merge pull request #4 from FluidityProject/fluidity-core/manualoverhaul201406

General manual overhaul in the system sections, including: