Some more minor copyediting/proofreading, good for both v0.12 and dev branches

Hi @spf13,

Sorry for my long absence.  I have been rather busy the past little while, but finally finding a little bit of free time to catch up on playing with Hugo.  :-)

Here is the latest PR for minor copyediting/proofreading to the docs.  Since no new content was added, the changes are good for both v0.12 and the upcoming v0.13.

Please note that the new tutorial "Creating a New Theme", i.e. `docs/content/tutorials/creating-a-new-theme.md` that was committed recently, does not show up in the side menu.  I tried to remedy that changing `[[menu:main]]` to `[[menu.main]]` similar to the syntax in `config.toml`, but now the following error shows up when compiling the docs:

    ERROR: 2014/12/08 unable to process menus for "Creating a New Theme"

The only way I could get "Create a New Theme" to appear in the side menu was to convert the front matter `creating-a-new-theme.md` from TOML to YAML, like the other Markdown pages in `docs/contents`.  However, as this seems to be indicative of a potential bug in Hugo's TOML parsing, I have decided to keep the front matter in TOML format to facilitate debugging of Hugo or one of its helpers.

Thanks!