Merge branch 'master' into 'master'

Add Mather app

[Mather](https://github.com/icasdri/Mather): Licensed GPLv3+. Straightforward build via `gradle`. Versioned as semver via git tags.

I've tested the build (via fdroidserver) with SDK version 25.1.7, Build Tools version 24.0.1, Android Platform version 23, Android Support version 23.2.1.

Relevant Submission Queue thread: https://f-droid.org/forums/topic/mather/

See merge request !1605

metadata/org.icasdri.mather.txt

@@ -0,0 +1,36 @@
+Categories:Science & Education
+License:GPLv3+
+Web Site:
+Source Code:https://github.com/icasdri/Mather
+Issue Tracker:https://github.com/icasdri/Mather/issues
+
+Auto Name:Mather
+Summary:A powerful expression-based calculator, unit converter, and computation engine
+Description:
+Mather is a powerful, simple-to-use, no BS expression-based computation engine.
+In addition to basic calculator arithmetic, it supports all of the following.
+Powered by Math.js.
+
+* Variables
+* User-defined functions
+* Easily-accessible math constants, e.g. 'pi'
+* Complex math expression evaluation
+* Trigonometric functions
+* Unit conversions (and more general unit arithmetic)
+* Complex numbers
+* Matrices
+* and more
+.
+
+Repo Type:git
+Repo:https://github.com/icasdri/Mather
+
+Build:0.2.3,203
+    commit=0.2.3
+    subdir=app
+    gradle=yes
+
+Auto Update Mode:None
+Update Check Mode:Tags
+Current Version:0.2.3
+Current Version Code:203