update .travis.yml

- restrict builds to master branch and release tags
- implement 'core' and 'results' test categories

scripts/run_tests.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+
+ROOTDIR="$(realpath "$(dirname "$0")/..")/"
+
+TESTS_CORE=(config cookies oauth text util)
+TESTS_RESULTS=(extractors)
+
+
+# select tests
+TESTS=()
+case "${GALLERYDL_TESTS}" in
+    core)    TESTS=( ${TESTS_CORE[@]}    );;
+    results) TESTS=( ${TESTS_RESULTS[@]} );;
+esac
+
+
+# transform each array element to test_###.py
+TESTS=( ${TESTS[@]/#/test_} )
+TESTS=( ${TESTS[@]/%/.py}   )
+
+
+# run 'nosetests' with selected tests
+# (or all tests if ${TESTS} is empty)
+nosetests --verbose -w "${ROOTDIR}/test/" ${TESTS[@]}