doc: use "Mroonga" as storage engine name

"Mroonga" is also used as MySQL storage engine name.
doc: add a missing space
Fix style
storage: fix a bug that no record returns with multiple column index

Redmine: #2651

Reported by foamcentime. Thanks!!
Reported by Naoya Murakami. Thanks!!

It is occurred when

  * columns indexed by multiple column index are used in order in WHERE and
  * a column indexed by multiple column index but it's not the first
    column is used in ORDER BY

See test cases in this commit for details.
doc install: add more descriptions to build from source
doc install: add more description about Groonga's development package
doc ja: remove garbage "<"
doc: use only GitHub for accepting problem reports

GitHub issues is easy to submit a report rather than our
Redmine. Because reporters don't need to create a new account for our
Redmine.

TODO:

  * Translate me!
travis: use released versions
travis: restrict the max concurrency
travis: remove needless "$"...
travis: fix wrong true literal
travis: disable allowing failure of building with system MySQL
travis: remove needless concurrency restriction for MariaDB bundled build
travis: install before test when MariaDB bundled build
travis: use the same test usage for MariaDB bundled build
travis: fix cmake option
travis: debug
travis: use MROONGA_ prefix to avoid variable name conflict
travis: disable concurrency build for MariaDB bundled build

    make[3]: warning: jobserver unavailable: using -j1.  Add `+' to parent make rule.