Add run_analysis.R

This script:
## 1. Merges the training and the test sets to create one data set.
## 2. Extracts only the measurements on the mean and standard deviation
##    for each measurement.
## 3. Uses descriptive activity names to name the activities in the data
set
## 4. Appropriately labels the data set with descriptive variable names.

## 5. From the data set in step 4, creates a second, independent tidy
data set
##    with the average of each variable for each activity and each
subject.