created settings reader

SettingsReader.java looks for a file called settings.txt in the
project's root folder. Settings are stored as key-value pairs of
strings. In settings.txt, each line which contains exactly one = sign is
interpreted as a key=value pair; each line with exactly no = signs is
interpreted as a key, and the value "true" is assigned to it
automatically. For now, the only setting used is "theorem-path", the
path to the directory where the theorems are stored. Theorem.java has
been updated to use the settings file for this, instead of using a
hard-coded value. If settings.txt cannot be found, SettingsReader.java
should complain and provide a default value for each setting.
default settings are only created once

The default settings are now stored in a final HashMap, so that they
don't have to be populated every time the settings file fails to read.
Also made it possible to manually ask to reload the settings file the
next time somebody asks for a setting.
added comments and JavaDoc

No code changes this time around! If this commit introduces a new bug,
the only response will be to assume a fetal position and cry oneself to
sleep.
Merge remote-tracking branch 'origin/master' into settings-file
Added an example settings.txt

Whitespace before the key, around the = sign, and after the value is
allowed but not necessary.
Merge remote-tracking branch 'origin/settings-file'