add unit tests and determine version from GIT_TAG (#66)

* add unit tests

* determine package version from git tag. closes #54.

* update readme

* bugfix

* update example

* update gitignore

test/unit/dependency_properties.cmake

@@ -0,0 +1,30 @@
+cmake_minimum_required(VERSION 3.14 FATAL_ERROR)
+
+include(${CPM_PATH}/CPM.cmake)
+include(${CPM_PATH}/testing.cmake)
+
+set(CPM_DRY_RUN ON)
+
+CPMAddPackage(
+  NAME A
+  GIT_TAG 1.2.3
+)
+
+CPM_GET_PACKAGE_VERSION(A VERSION)
+ASSERT_EQUAL(${VERSION} "1.2.3")
+
+CPMAddPackage(
+  NAME B
+  GIT_TAG v2.3.1
+)
+
+CPM_GET_PACKAGE_VERSION(B VERSION)
+ASSERT_EQUAL(${VERSION} "2.3.1")
+
+CPMAddPackage(
+  NAME C
+  GIT_TAG v3.1.2-a
+)
+
+CPM_GET_PACKAGE_VERSION(C VERSION)
+ASSERT_EQUAL(${VERSION} "3.1.2")