Allow for sync XHR requests.
Ignore vim-generated files.
Moved toggleButtons to euskalbar.ui.
Implemented common API for queries and combined queries.

Dictionaries should now provide the necessary properties and methods as part of
their own implementation.
Rather pass term to the querying function.
Ensure we always return a string.
Always return a value, check for strict value.
Simplified function, always return a value.
Improved common API to cover more possible situations.
Added simple logging utility function.
Implemented a plugin-like system for dictionaries.

This commit has invasive changes, including:

- Each dictionary lives in its own script file. Dictionaries implement a set of
  internal APIs that provide all the required information for queries and
  combined queries (metadata, URL and parameter retrieval, scrapping functions,
  etc.).
- `euskalbar.dicts` has a fixed list of available dictionaries that will be
  loaded at init time. Any name defined there will be considered as a script
  file that should be loaded from the `dicts` directory.
- Lots of dictionary-specific code has been removed.
- IDs used in the UI elements have been unified, now all mentions to 'euskalbar'
  are lower-cased.

This is not complete though, there are several parts that need to be reviewed or

improved:
- Some combined query scrapping functions still need an implementation.
- Subqueries haven't been tested.
- UI elements can and should be generated dynamically based on the list of
  available dictionaries.
- More dictionary-specific code needs to be removed from the main script files.

Fixes #22.
Let's be safe when toggling non-existent dictionaries.
Moved selectionText to euskalbar.ui.
Removed dead code, small readability improvement.
Removed dead code.
Micro optimization.
Use native JS objects for identifying functions and arrays.

There's a bug in older versions of Firefox that always returns
`[xpconnect wrapped native prototype]` if the `toString` function is called from
the window context.

More information at https://bugzilla.mozilla.org/show_bug.cgi?id=684601
Explicitely set supported language pairs.
Fixed homepage URL.
Load dictionaries into a language pairs structure.