Adding output data files.

Adding the following files:
1. activity.txt - This file has activity data with descriptive activity
name.
2. activity_measurements.txt - This file stores data set that has
selected columns of the original data set and have descriptive names.
3. activity_measure_mean.txt - This file stores the final tidy data set
that has summarized mean value of measurements for each subject for each
activity and for each feature.